Surface cleaning and repair of the surface of, for example, buildings, vehicles, and energy collection devices, are time-consuming and costly, and a surface with an inherent repellency of water, oil, and dirt can be a significant advantage. Surface wetting is governed by surface-energy parameters between the surface and the contacting liquid or solid surface. Where the sum of the free surface energies between materials components is very low, adhesion between these materials is weak. Hence, it is generally beneficial to lower the free surface energy of an edifice if one hopes to ignore its cleaning and repair. Non-stick materials, such as perfluorinated hydrocarbons have very low surface energies such that few materials adhere to Teflon®. The wetting of these low surface energy materials is reflected in the contact area that is observed between the surface of the low surface energy solid and a wetting material. The interactions between these materials generally result from van der Waals forces.
Nature diminishes the interaction of a surface of a solid and water without resorting to materials with surface energies as low as Teflon®. This is achieved by reducing the amount of the surface that contacts the water. For example, lotus leaves, cabbage leaves, and various fruits are covered by small wax bumps that reduce the van der Waals contact area presented to a water droplet that forms due to its high surface tension, which significantly reduces the adhesion of the droplets to the surface. These superhydrophobic textured surfaces display water contact angles that are in excess of 150° and display low sliding angles, which is the critical angle from horizontal of the inclined surface where a water droplet of a defined mass rolls off the inclined surface. This “Lotus effect” provides a self-cleaning surface, as contact water droplets adhere to dust particles and, to a much lesser degree, to some oils that are poorly adhered to the surface, which allows the “dirt” to be carried away as the water droplet rolls off the surface. Most oils are not readily removed from such hydrophobic surfaces as the enlarged surface area increases the effective van der Waals interface and the Lotus-effect surface does not repel oils that cannot interact more favorably with water than the textured surface.
Oil repellent surfaces are an engineering challenge because the surface tensions of oily liquids are usually in the range of 20-30 mN/m. Hence, the essential criterion, for having a surface with superoleophobicity, is to maintain oil drops in a Cassie-Baxter (CB) state, one where vapor pockets are trapped underneath the liquid. The CB state is dependent on the surface's structure and the surface energy of the material. If the structure and surface area are insufficient, the meta-stable energetic state is transformed into Wenzel state. The geometric features that allow this state have re-entrant structures, such as mushroom heads, micro-hoodoos, or horizontally aligned cylindrical rods. A re-entrant structure implies that a line drawn vertically, from the base solid surface through the geometric feature, must proceed through more than one solid interface of that feature.
To achieve surfaces that display high or superhydrophobicity and high or superoleophobicity, nanoparticles can be spray deposited to form the textured surface. For example, Lin et al., Surf. Coat. Tech., 2006, 200, 5253-58, discloses the spraying of a dispersion of spherical TiO2 or tetrapod-like ZnO nanoparticles with a fluorinated binder onto a substrate to achieve contact angles as high as 161.4° with water and the demonstration of oil repellency. Ogihara et al., Langmuir 2012, 28, 4605-8 discloses superhydrophobic paper by spraying a suspension of dodecylsilyl-functionalized silica nanoparticles in alcohol onto paper and achieved contact angles of up to 155° without a binder. Mertaniemi et al. RSC Adv., 2012, 2, 2882-2886 discloses the spray deposition of tridecafluoro-1,1,2,2-tetrahydrooctyl)silyl-functionalized nanofibrillated cellulose microparticles in ethanol onto glass to yield a semi-transparent coating with a contact angle of 163° upon drying. Li et al., Appl. Surf Sci., 2012, 261, 470-2 discloses superhydrophobic paper, prepared by spraying a suspension of octadecylsilyl-functionalized 50 nm silica nanoparticles on paper, where the transparent coating displayed a contact angle of 163°.
Although superhydrophobic and superoleophobic surfaces have been produced there remains a need for the formation of superhydrophobic surface for transparent substrates and methods of producing transparent superhydrophobic coatings. Such coatings can be useful for windows and other transparent devices, particularly those whose access for cleaning is difficult or dangerous. Furthermore, a simple method of depositing a superoleophobic coating to a surface is desirable.